This invention relates to a power brake unit and more particularly to a power brake unit for an automotive vehicle comprising a master cylinder and a booster utilizing the pressure difference between a vacuum and atmospheric pressure.
From U.S. Pat. No. 3,222,868, a power brake unit is known comprising a vacuum casing including a cup shell and a cover shell, a working piston movable therein including a diaphragm and a push rod. A master cylinder of a hydraulic brake system is fastened to the front end of the cup shell of the vacuum casing. Bolts are provided in the cover shell which serve to secure the whole unit--power brake unit and master cylinder--to the vehicle.
In an arrangement of this type, the parts of the vacuum casing are conventionally made of sheet steel and the master cylinder is made of a gray cast iron casting or a light metal casting. To increase the mechanical load-carrying ability, the casing parts may be reinforced. When the brake pedal is depressed, the force transmitted from the brake pedal and the booster force are effective on the push rod. The sum of the forces acts on the piston of the master cylinder, the resistance of the compressed hydraulic fluid in the master cylinder requiring, however, the presence of a force component acting in the opposite direction to keep the master cylinder approximately in its position. This reaction force is transferred from the master cylinder via the parts of the vacuum casing to the splashboard to which the holding device for the pedal device is conventionally fastened. The reaction force causes an extension of the vacuum casing in an axial direction and, consequently displacement of the master cylinder, which results in an additional lost travel of the brake pedal.
Moreover, the known arrangements are unable to comply with the vehicle manufacturers' demand for a lowest possible weight of brake systems without the safety and operability of the power brake unit being impaired. Substituting light-metal parts for the sheet-steel parts is not reasonable because light metals have a lower modulus of elasticity which could be compensated for only by a greater geometrical moment of inertia. Neither is this design practical from the point of view of material costs.